CLEVELAND — It began with a swap: one boy’s cellphone for another’s replica of a Colt pistol.

One of the boys went to play in a nearby park, striking poses with the lifelike, airsoft-style gun, which fired plastic pellets. He threw a snowball, settled down at a picnic table and flopped his head onto his arms in a perfect assertion of preteen ennui, a grainy security video shows.

Then, with the gun tucked away, he walked to the edge of the gazebo. He might have been wandering aimlessly, or he might have been attracted by the sight of a squad car barreling across the lawn.

Seconds later, the boy lay dying from a police officer’s bullet. “Shots fired, male down,” one of the officers in the car called across his radio. “Black male, maybe 20, black revolver, black handgun by him. Send E.M.S. this way, and a roadblock.”

But the boy, Tamir Rice, was only 12. Now, with the county sheriff’s office reviewing the shooting, interviews and recently released video and police records show how a series of miscommunications, tactical errors and institutional failures by the Cleveland police cascaded into one irreversible mistake.

And in death last November, Tamir joined Michael Brown, a teenager fatally shot by a police officer in Ferguson, Mo., and Eric Garner, a Staten Island man who died after being placed in a chokehold by an officer, as touchstones for protests of police violence against unarmed black people across the nation. Their names were chanted by demonstrators again on Monday in Martin Luther King Jr. Day marches.

Because of multiple layers in Cleveland’s 911 system, crucial information from the initial call about “a guy in here with a pistol” was never relayed to the responding police officers, including the caller’s caveats that the gun was “probably fake” and that the wielder was “probably a juvenile.”

What the officers, Frank Garmback and his rookie partner, Tim Loehmann, did hear from a dispatcher was, “We have a Code 1,” the department’s highest level of urgency.

When the officers raced into action, they took a shortcut that pointed their squad car straight into the park, pulling up so close to Tamir that it made it difficult to take cover, or to use verbal persuasion or other tactics suggested by the department’s use-of-force policy.

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A family photo of Tamir Rice from the fall of 2014.

Within two seconds of the car’s arrival, Officer Loehmann shot Tamir in the abdomen from point-blank range, raising doubts that he could have warned the boy three times to raise his hands, as the police later claimed.

And when Tamir’s 14-year-old sister came running up minutes later, the officers, who are white, tackled her to the ground and put her in handcuffs, intensifying later public outrage about the boy’s death. When his distraught mother arrived, the officers also threatened to arrest her unless she calmed down, the mother, Samaria Rice, said.

Officers Garmback and Loehmann did not check Tamir’s vital signs or perform first aid in the minutes after he was shot. But Officer Garmback frantically requested an emergency medical team at least seven times, urging the dispatcher to “step it up” and to send medical workers from a fire station a block away. It would be eight minutes before they arrived.

The shooting fit into a broader history of dysfunction at the Cleveland Division of Police. Two weeks after Tamir’s death, the Justice Department released a scathing report accusing the department of a pattern of excessive force for which officers were rarely disciplined, and pressed the department to accept a federal monitor. Just a year before, in 2013, an investigation by the state attorney general found “systemic failure” in the department.

It also highlighted shortcomings in the department’s vetting process for recruits. Police records show that Officer Loehmann was hired without a review of his file at a previous department, where he resigned after suffering a “dangerous loss of composure” during firearms training.

The Cleveland police department and mayor’s office declined to comment for this article.

For Cleveland residents, the shooting highlighted another longstanding problem: The department’s community policing programs had been whittled down to a token effort, a result of cuts a decade earlier that might well have made a life-or-death difference to Tamir. A sign on a telephone pole yards from where he was shot down still advertises a police mini-station in the nearby recreation center where he played basketball. The station is long gone.

“If there was one there,” Councilman Jeffrey Johnson said, “he would have known Tamir, because Tamir was a regular, and he would have heard the call and gone out there and said, ‘Tamir, what are you doing?’”

A Real-Looking Toy

Before leaving his mother’s apartment on that gray Saturday, Nov. 22, Tamir went through one of her drawers to find a plaything: her cellphone.

He was known as a boisterous, friendly boy. At school, where he had a good attendance record, Tamir was often in trouble, classmates said, mainly for his pranks: He was deft with a whoopee cushion and liked to reseal his empty milk carton to tempt the unsuspecting.

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A timeline of what happened after Tamir Rice, a 12-year-old boy, was killed by a police officer in Cleveland in November 2014.Published OnJan. 22, 2015

“He was bad, but like in a funny way,” said Deovaunté Hotstetter, a 10-year-old schoolmate. “I can’t remember what was so funny, but there was cussing in it.”

Deonte Goldsby, 21, a relative, said Tamir, the youngest of four, would take care of his smaller cousins at family gatherings, chasing them or playing with their action figures and dolls. With adults, he was well-mannered, using ma’am and sir and offering to fetch sodas from the refrigerator.

Cudell Commons, where Tamir was shot, was the geographic center of his daily life. The park is flanked on one side by the recreation center where Tamir, who stood 5-foot-7, played basketball, boasting, “You can’t check me!” when he scored. On the other side stood his school, Marion C. Seltzer Elementary, where the calendar is printed in five languages and the bulletin boards teach children to distinguish stereotypes from reality.

An older friend told Tamir that he could take the cellphone, whose service was locked, to a store and make it work, Tamir’s mother said. Tamir asked if he could hold the friend’s airsoft pistol while he was gone. He seemed delighted in the novelty of the replica.

“His mother didn’t allow him around guns whatsoever, toy guns, water guns,” Mr. Goldsby said. “She knows about things like this. She knows that somebody would mistake it for a real gun.”

In this case, the replica was a few years old, and the orange safety tip, intended to distinguish it from a pistol that fired real bullets, had been removed or had fallen off. Just as his mother had worried, Tamir wound up in the park waving what looked very much like a real weapon.

A Lifelong Police Interest

The 911 caller was calm, pausing to exchange pleasantries with the dispatcher before getting to the point: A male in Cudell Commons was pointing a pistol at people and scaring them. The gun was “probably fake,” he said twice before signing off, and its wielder was “probably a juvenile.”

Officer Garmback, 46, who had joined the force in 2008, was at a nearby church when the call came. With him was his partner, Officer Loehmann, 26, hired just eight months before.

Officer Loehmann had grown up in Parma, a largely white suburb of Cleveland, but he commuted 30 minutes to an all-male, Roman Catholic high school on the city’s east side, Benedictine, where many of the students were minorities.

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The airsoft gun that Tamir Rice was playing with at the time he was killed.CreditMark Duncan/Associated Press

People who knew Officer Loehmann there recalled him as quiet and serious, active in the band and the German Club. The Rev. Gerard Gonda, the school’s president, said Mr. Loehmann had a solid record at Benedictine, where as a junior he was in Father Gonda’s theology class. “He had a very low-key personality, and I would say kind of a gentle personality,” Father Gonda said.

The Rev. Anselm Zupka, who taught Officer Loehmann at Benedictine and was also his confirmation sponsor at his local parish, said “Timmy” had embraced his Catholicism to an extent that Father Zupka suggested to him that he might want to enter the monastery.

But the teenager had other ideas. “He was always interested in police work, because that’s what his father did,” Father Gonda said.

Officer Loehmann had long wanted to emulate his father, Frederic, who served in the New York Police Department for 20 years before becoming a federal marshal. So in 2011, he earned a bachelor’s degree in criminology and sociology from Cleveland State University, according to his personnel file, and the next year, he went to work for the police in Independence, Ohio.

But there, according to police records, he had emotional problems related to a girlfriend. At a shooting range, he was “distracted and weepy,” a supervisor said. One of his supervisors concluded that Officer Loehmann “would not be able to substantially cope, or make good decisions,” during stressful situations. After six months, the department allowed him to resign.

Officer Loehmann stayed in the Cleveland area, where he took private security jobs. He continued to apply for local law enforcement jobs but was not hired until the Cleveland police gave him a chance, in March 2014. The department never reviewed his Independence personnel file.

Officer Loehmann did well, graduating from the Cleveland Police Academy with a score of 98.8. He was assigned to a district on Cleveland’s west side, which included the poor, blighted neighborhood around Cudell Commons.

Episodes of Abuse

By the time Officer Loehmann was hired, the department was already struggling with a host of problems that had begun at least a decade before.

In 2004, city leaders laid off 250 officers to help close a budget gap. That trimmed the force 15 percent, to about 1,500 officers, seriously hurting community policing and closing mini-stations.

Over the next two years, the city’s violent crime rate leapt by double digits. It has since declined from that peak, but the city is still more violent than it was in 2004, according to F.B.I. data, even as violent crime has continued to drop across Ohio and the country.

As the police department was shrinking, it came under increasing criticism for excessive use of force. The Justice Department began an investigation prompted by police shootings that led to an agreement in 2004 calling for the city to tighten its guidelines for the use of force and to improve its documentation of those incidents. But many reforms were not maintained, according to the recent Justice Department report.

Episodes of abuse continued to surface. In 2011, a helicopter video captured police officers kicking Edward Henderson in the head even though he was spread-eagled on the ground. None of the officers admitted to wrongdoing, and none were fired, though the video showed them “kicking his head like a football,” said David Malik, a prominent civil rights lawyer who won a $600,000 settlement for Mr. Henderson, who suffered a broken facial bone.

Mr. Malik said the city’s discipline and arbitration system heavily favored officers, making it difficult to punish misconduct. “It’s a culture of no consequences,” said Mr. Malik, who has filed or investigated potential lawsuits against the Cleveland police on more than 100 occasions.

Nearly two years after the assault on Mr. Henderson, more than 60 police cruisers and one-third of the city’s on-duty force engaged in a high-speed chase after officers mistook a car’s backfiring for gunfire. It ended when officers killed the two unarmed occupants by firing 137 rounds into their vehicle.

The episode prompted an investigation by the state’s attorney general, Mike DeWine, a Republican, that found systemic breakdowns in communication and supervision in the department.

“When everybody violates the rules,” Mr. DeWine said in an interview, “the cops are not the problem. You’ve got a culture problem, you’ve got a command-and-control problem, you’ve got a management problem, which goes way past those guys.”

The deadly chase also spurred calls for a new Justice Department investigation. Released in December, that study found a pattern of excessive force, suggesting that the police were often hostile with residents and were rarely held accountable for misconduct.

“Officers use excessive force against individuals who are in mental health crisis or who may be unable to understand or comply with officers’ commands, including when the individual is not suspected of having committed any crime at all,” the report said.

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A memorial near where Tamir Rice played basketball.CreditMichael F. McElroy for The New York Times

Cleveland and the Justice Department have agreed to work toward a consent decree that would tighten use-of-force policies and subject the department to oversight by a monitor.

Critics of the force cite hiring standards that require only a high school diploma or equivalent at a time when many big-city departments require some college, and its failure to adequately analyze use-of-force and arrest data in ways that have become standard at many departments.

Detective Steve Loomis, the president of the largest local police union, disputed the idea that the system for resolving complaints against officers favored the police. But he acknowledged problems in the department, including what he said was understaffing and low compensation that forced many officers to take second jobs to make ends meet.

Waiting for Answers

In the weeks since Tamir’s death, the city and its police department have come under mounting pressure to explain not only the shooting, but also its aftermath, with the officers failing to provide first aid as Tamir lay bleeding. Not until an F.B.I. agent who happened to be nearby arrived four minutes after the shooting did anyone tend to the boy.

Though the department’s use-of-force policy requires officers to “obtain necessary medical assistance” for injured people, it does not explicitly call for them to perform first aid. Walter Madison, a lawyer for Tamir’s mother, said it would be ludicrous to believe that officers would not immediately perform first aid on a wounded comrade.

Henry Hilow, a lawyer representing Officer Loehmann, said the officers had followed protocol by calling for E.M.S., saying, “They were doing the best they could to get medical attention” for Tamir. He also defended the officers’ tactics in the moments before the shooting, saying they had positioned their cruiser to prevent Tamir from running into the recreation center, where they thought he might endanger people. They tried to stop farther away, but the car skidded, Mr. Hilow said.

Echoing the defense of the police department after the shooting, Mr. Hilow also said the officers had seen Tamir pull the pellet gun out of his waistband moments before Officer Loehmann shot him, an account that is difficult to verify with the low-quality security video. He said the officers had shouted at Tamir to drop the gun and show his hands before their squad car came to a stop.

The department has begun taking some steps to address its problems. It says it will review personnel files of all new hires. A city inquiry may also examine the dispatch system, in which, Detective Loomis said, the person who took the 911 call did not relay the caller’s caveats to the dispatcher.

Yet Mayor Frank G. Jackson, a Democrat in his third term, has insisted there is no “systemic failure” in the department, and has steadfastly resisted calls for the resignation of two top advisers who oversaw the department during the period studied by both the state and the Justice Department.

Ms. Rice, 38, is awaiting explanations, and an apology. “Nobody has come to knock on my door and told me what happened,” she said. “Somebody has to be held accountable.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Many Errors by Cleveland Police, Then a Fatal One. Order Reprints | Today’s Paper | Subscribe